In hoc signo vinces

Labarum of Constantine
Reconstructed from the coin shown below.
The three circles probably had portraits of rulers.
Image wikimedia

Constantine I 307-337 AD. Æ en:Follis (2.97 gm)
Struck 337 AD. Constantinople mint.
CONSTANTI-NVS MAX AVG. SPES PVBLICA. CONS in exergue.
labarum, with three medallions on drapery and crowned by a christogram, spearing serpent.
Image wikimedia

“Then will appear the sign of the Son of Man in heaven. And then all the peoples of the earth will mourn when they see the Son of Man coming on the clouds of heaven, with power and great glory. And he will send his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of the heavens to the other."
Matthew 24:30-31 NIV

Ichthys inscription from Ephesus
Image wikimedia

Christ talks in Matthew about the sign of the Son of Man but we do not know what sign is in question. Among the earliest symbols for Christ found in excavations in Israel and in catacombs was the fish, in Greek Ichthys.  "Ίησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ", (Iēsous Christos, Theou Yios, Sōtēr) Jesus Christ, Son of God, Saviour.

It is numismatic fact that Constantine the Great knew the Chi Rho symbol and this must have been the sign painted on the helmets and shields of his legionaries in the crucial Battle of Milvian bridge on October 28 312. Before  that eventually led to his rise to power as the sole Emperor of Rome and to events that made Jesus Christ the divine ruler of Rome and Constantinople.

Image MudPreacher

However, today world knows better cross than labarum as the sign of the Son of Man. In early church it was such a cruel tool of execution that it was not used as religious symbol. However, after Constantine the Great forbid crucifixion its horrors were forgotten and gradually during the 4th and 5th centuries it gradually became the dominant symbol of Jesus Christ in art.